Most functions work on the typical example their author had in mind. Bugs show up on the unusual inputs, called edge cases. Before testing a function, run through this list:

Edge	Try
Empty	`""`, `[]`, `{}`, 0 items
One	a single character, a list of one
Zero and negatives	0, -1, a list of negatives
Boundaries	exactly at a limit: 49, 50 and 51 if 50 is the pass mark
Duplicates	repeated values, ties
Order	already sorted, reversed
Case and spaces	capitals, leading and trailing spaces
Large	a long list, a big number

You don't need all of these for every function, but each one you skip is a place a bug can hide. Notice that the course's own tests have been doing this all along.